An important component of disposable absorbent articles such as diapers is an absorbent core (layer) including water-absorbing polymeric material, typically hydrogel-forming water-absorbing polymeric material, also referred to as absorbent gelling material, AGM, or super-absorbent polymer, SAP. This polymeric material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness. Especially useful water-absorbing polymeric materials are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g. sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. Traditionally, these water-absorbing polymers are incorporated into absorbent structures with cellulose or cellulosic fibres to provide an absorbent structure wherein the water-absorbing polymers can swell and absorb large quantities of urine with a reduced risk of gel-blocking and to ensure the right gel-bed porosity or permeability, and also to ensure the absorbent structure is stable in use or during transport. In recent years, the focus has been to make thinner absorbent structures. Hereto, it has been proposed to reduce or eliminate these cellulose fibres from the absorbent structures. However, the absorbent structure may loose part of its mechanical stability in use without the presence of cellulose fibres, and the water-absorbing structure may suffer from gel-blocking.
It has thus been proposed to use other matrix materials, in smaller quantities or volumes, such as fibrous adhesives, to provide absorbent structures that have the required permeability/porosity, and reduced gel-blocking, and that form a stable structure in use or transport. For example thermoplastic adhesives are used to form a matrix for the water-absorbing particles and to immobilise these.
In recent years there has also been ongoing development of water-absorbing polymers with improved performance, such as improved capacity and improved permeability. Various coated water-absorbing polymeric materials have been proposed to have an improved performance.
The inventors have now found that surface-modified, e.g. surface-coated, water-absorbing polymeric particles may not perform satisfactory when incorporated into an absorbent layer with certain thermoplastic adhesive matrix materials; in particular the absorbency and/or absorbency speed may be reduced, in particular in time after extended storage periods. They found that this may be due to the interaction of certain components in the adhesive thermoplastic matrix materials with the surface of the surface-modified water-absorbing polymers, rendering their surface more hydrophobic, and thereby reducing the affinity of these surfaces for hydrophilic materials like urine, and thus reducing the absorbency of urine by the water-absorbing polymers.
The inventors have now found improved absorbent articles that incorporate surface-modified water-absorbing particles, e.g. including an additional (hydrophilic) surface modification, e.g. a coating (of for example film-forming and/or elastic polymers) with a thermoplastic adhesive material that does not negatively impact the properties, e.g. hydrophilicity, and the performance of the surface-treated water-absorbing polymeric particles. These thermoplastic adhesive materials may thus be used to make (thin) absorbent layers and articles that may even be absorbent cellulosic fibre-free, but having nevertheless a good gel-bed porosity, whilst the surface-modified water-absorbing polymeric particles maintain an excellent absorbent capacity, and importantly, absorbent speed.